What arduino is?
If it is an old one with ftdi you can change the com port in the device manager and probably will work again. Also as alternative I suggest you to test my mod version that will try to fix the issue when it happens: http://arduino.cc/forum/index.php/topic,118440.0.html
It is like I am selling the mod but in fact I am pretty sure 99% of the problems in this forum are because installation and this issue with serial ports 